1. Field of the Invention
The present invention relates to a cathode ray tube having a substantially flat face panel and a method of manufacturing the same.
2. Description of the Related Art
Generally, a color cathode ray tube is provided with a vacuum envelope having a glass face panel and a glass funnel. A phosphor screen having three-color phosphor layers is formed on the inner surface of the effective area of the face panel, and an electron gun is arranged in the neck of the funnel. Three electron beams emitted from the electron gun are deflected by the magnetic field generated by a deflector mounted on the outside of the funnel, and scan horizontally and vertically on the phosphor screen through a shadow mask, thereby displaying a color image.
The face panel of a color cathode ray tube having this configuration generally includes a substantially rectangular effective area and a side wall erected along the peripheral portion of the effective area. The face panel is formed with inner and outer surfaces curved so differently that the central portion of the effective area is thinner than the peripheral portion thereof in order to secure a sufficient strength to resist the atmospheric load applied to the vacuum envelope.
Generally, the outer surface of the effective area is formed with such a curvature that the height with respect to the sealed surface between the face panel and the funnel is greatest at the central portion of the effective area and lower toward the peripheral portion. Specifically known face panels include the one with the outer surface of the effective area having a spherical curvature, the one having a cylindrical outer surface with a substantially infinitely large radius of curvature along the vertical axis and a curvature along the horizontal long axis, and the one having a curved outer surface expressed by a high-order polynomial.
With respect to the shape of the outer surface of the effective area of the face panel, the recent trend is toward the flattening in order to improve the visibility. Depending on the curved geometry of the outer surface of the effective area of the face panel, a generally known method of indicating the flatness of the effective area includes the index R. The index R is given as the ratio of the average radius of curvature of the corners determined by the difference (the fall of the corners) between the height of the central portion of the face panel and the height of the corners of the face panel to the diagonal length of the effective area multiplied by a factor of 1.7. In the case where the flatness expressed by this index R remains the same, the fall at the corners is the same for any shape of the curved outer surface of the effective area, and though somewhat depending on the geometry of the curved surface, the feeling of flatness of the effective area of the face panel is substantially equal.
With the increase of the flatness of the face panel, however, the atmospheric strength of the glass vacuum envelope decreases. The flatness of the outer surface of the effective area, therefore, is at most about 2.0R even for a large cathode ray tube.
On the other hand, various shapes are available for the inner surface of the effective area of the face panel. The inner surface of the effective area, however, is often formed in the same type of curvature as the outer surface of the effective area so that the effective area is thinnest at the central portion thereof and thicker toward the peripheral area in order to maintain the atmospheric strength required of the glass vacuum envelope.
In recent years, the atmospheric strength of the glass vacuum envelope has improved due to an improved design accuracy of the face panel and an improved performance of the reinforcing band to such an extent that a predetermined strength is secured even with a flattened face panel. In the case where the inner and outer surfaces of the face panel are configured of the same type of curvature as described above, however, a still higher strength of the vacuum envelope against the atmospheric pressure is required if the effective area of the face panel is to be flattened more. This in turn requires reinforcing by increasing the glass thickness greatly or attaching a reinforcing film on the outer surface of the effective area of the face panel at the sacrifice of a remarkably higher cost.
On the other hand, there exists a cathode ray tube comprising a face panel having a substantially flat outer surface of the effective area. In this cathode ray tube, however, the inner surface of the face panel is formed by a combination of curved surfaces like the well-known face panel. For this reason, the vacuum envelope is reinforced by thickening the effective area of the panel or attaching a reinforcing film on the outer surface of the effective area of the face panel in order to secure the atmospheric strength of the vacuum envelope. This leads to a considerably higher cost as in the above-mentioned case.
With the color cathode ray tube, the shadow mask is configured of a substantially rectangular flat mask body about 0.1 to 0.3 mm thick and a substantially rectangular frame fixed on the peripheral portion of the mask body. The effective surface of the mask body is opposite to the phosphor screen and the effective surface is formed with a number of apertures allowing the electron beams to pass therethrough.
Generally, the effective surface of the mask body is shaped in conformance with the inner surface of the effective area of the face panel and has at least a curved central portion protruding toward the phosphor screen. The shape of the curved surface conventionally used includes a cylindrically curved surface having a predetermined curvature along the horizontal axis and a substantially infinitely large radius of curvature along the vertical axis or a curved surface expressed by a high-order polynomial.
Regardless of the shape of the curved surface of the shadow mask, the electron beam apertures of the shadow mask and the phosphor layer are required to be in specified relative positions in order to assure the accurate landing of the electron beams on the phosphor layers. The same relative positions are always required to be maintained through the whole operation of the cathode ray tube. In other words, the distance between the shadow mask and the phosphor screen must always be within a predetermined tolerance.
The amount of the electron beams that reaches the phosphor screen through the electron beam apertures of the shadow mask, however, is not more than one third of all the electron beams emitted from the electron gun, and the remaining electron beams bombard the shadow mask. The electron beams that have thus bombarded the shadow mask are converted into thermal energy to heat and expand the shadow mask.
The thermal expansion of the shadow mask increases the displacement of the beam landing and the deterioration of the color purity. The magnitude of the mislanding caused by the thermal expansion of the shadow mask is greatly varied with the image pattern displayed and the time during which an image pattern is sustained. Especially in the case where a locally high-luminance image pattern is displayed, the local doming of the shadow mask occurs so that the mislanding of the electron beam is caused within a short time resulting in a great displacement of the electron beam. The mislanding is most conspicuous in the case where the doming of the shadow mask occurs at a portion located toward the center from the horizontal end of the effective surface of the shadow mask by about one third of the horizontal length.
The two methods of forming the curved surface of the shadow mask include using the press work and (2) applying a tension. In the method using the press work, a planar mask plate (flat mask) made of a thin metal with a number of electron beam passage apertures is subjected to plastic deformation in the press. This method is used mainly for forming a spherical surface or a curved surface expressed by a high-order polynomial, as described above.
The second method of forming the curved surface of the shadow mask under tension is used for producing a cylindrical surface with a predetermined radius of curvature along the horizontal axis and a substantially infinitely large radius of curvature along the vertical axis. In this method, a planar plate of a thin metal with a number of electron beam passage apertures is arranged along the frame. The frame has a mask-mounting surface curved along the horizontal axis with a substantially infinitely large radius of curvature along the vertical axis. This mask plate is fixed to the frame under a tension applied along the vertical axis of the mask plate.
As described above, the curved surface of the shadow mask has been flattened and has an increasingly larger radius of curvature with the flattening of the face panel. With the increase in the curvature of the shadow mask, the strength of holding the curved surface of the shadow mask is reduced. As a result, the effective surface of the shadow mask is easily deformed under a shock or other external forces applied to the color cathode ray tube. Also, in the case where the color cathode ray tube is exposed to a vibration, the shadow mask is liable to develop a resonance (howling). In either case, the color purity of the displayed image is deteriorated.
The strength of holding the curvature of the flattened shadow mask can be improved by increasing the thickness of the shadow mask. An increased thickness of the shadow mask, however, makes it difficult to form the electron beam passage apertures by photoetching and difficult to obtain beam passage apertures with desired shape and size. Further, the cost of the material of the shadow mask increases.
As a measure for improving the curved surface-holding strength, a method is conceivable in which the shadow mask is mounted under a tension applied along the direction of the vertical axis having an infinitely large radius of curvature. In this case, however, the requirement of applying a very large tensile force to the shadow mask necessitates a very high strength of holding the shadow mask. As a result, the production cost of the color cathode ray tube increases. At the same time, the increased frame weight greatly increases the whole weight of the cathode ray tube.